1. Field of the Invention:
The present invention relates to an optical switch for selecting alternate light paths between or optically coupling and decoupling optical fibers for use in optical fiber communications.
2. Description of the Relevant Art:
One conventional optical switch, as disclosed in Japanese Laid-Open Patent Publication No. 56(1981)-107201, for example, comprises a rectangular prism positioned at the entrance/exit ends of three optical fibers arranged side by side in one plane. The rectangular prism is reciprocally movable in that plane in linear directions normal to the optical axes of the optical fibers, so that the optical fibers can be optically coupled and decoupled.
Japanese Laid-Open Patent Publication No. 55(1980)-142304 discloses, as shown in FIG. 21 of the accompanying drawings, three juxtaposed optical fibers 101, 102, 103 and a rectangular prism 104 coupled to a rotatable shaft 110 coaxial with the central optical fiber 101. The central optical fiber 101 can be optically coupled to either the optical fiber 102 or the optical fiber 103 by turning the rectangular prism 104 through 180.degree. on and about the shaft 110 in a plane normal to the plane in which the optical fibers 101, 102, 103 lie.
With the former optical switch, since the rectangular prism is linearly reciprocally moved perpendicularly to the optical fibers for light path coupling and decoupling, the inertial force which is applied to the rectangular prism when it is stopped is oriented in the same direction as the direction in which the rectangular prism is adjusted with respect to the optical axes of the optical fibers. Therefore, the adjusted position of the rectangular prism may be subjected to an error depending on the way in which the optical switch is used. For example, if the prism is displaced .DELTA.d from its adjusted position, then the distance between the axis of a light ray incident upon the prism and the axis of a light ray reflected from the prism will vary by .DELTA.d.times.2. Depending on how the prism is mounted, the prism is affected by gravity, and it takes different times to switch from one light path to the other and to switch from the other light path back to the one light path.
The latter optical switch takes a relatively long switching time and hence has a low switching speed since the prism 104 is required to turn 180.degree. for light path coupling and decoupling.
In each of the above conventional mechanical optical switches, because the entrance/exit end surfaces of the optical fibers are disposed parallel to the entrance/exit end surface of the prism, the exit end surfaces of the optical fibers and the prism tend to be affected by reflected light rays.
FIGS. 22(a) and 22(b) of the accompanying drawings show an optical switch disclosed in Japanese Patent Publication No. 62(1987)-56490. The disclosed optical switch includes four juxtaposed optical fibers 201, 202, 203, 204 lying in one plane, the fibers being disposed in coaxial confronting relation in two pairs. A polygonal prism is positioned so as to face the entrance/exit ends, i.e., input/output terminals, of these optical fibers. By turning the polygonal prism from the position of FIG. 22(a) to the position of FIG. 22(b), the optical fibers 201, 204 are optically decoupled and the optical fibers 201, 202 are optically coupled, and at the same time, the optical fibers 203, 202 are optically decoupled and the optical fibers 203, 204 are optically coupled. The polygonal prism has however its own disadvantages. For example, it causes a loss of light energy due to interfacial reflections. The polygonal prism is heavy and hence poor in its response to switching commands on account of large inertia thereof. As a result, the switching speed of the optical switch is low.